1. Field of the Invention
The present invention comprises an apparatus and method for lens casting without using a gasket.
2. Background Art
Most lenses today are made by molding plastic because of its light weight, density, refractive index, and impact resistance. To form a lens, two molds, often referred as a front mold and a back mold in the art of lens making, are used. Each mold has a facing inside surface. When these two molds are properly positioned at a desired distance and rotational orientation to each other, their facing inside surfaces are a negative image of the surfaces of the lens to be formed. A closure member is used to necessarily seal the cavity. Then a fluid lens-forming mixture, normally a liquid monomer, is placed and contained in the cavity defined by the two molds and the closure member. Once the fluid lens-forming mixture is in the cavity, it is cured to form a hardened polymeric lens taking the shape of the molds.
Two kinds of closure member are now used in making the lenses. The first kind of closure member is a gasket, which has been used widely in the art. Currently available gaskets are known as T-gaskets. As shown in FIG. 1 in cross-section, a T-gasket G has a bore B and two ends that each complementarily receives a respective mold M. Once the molds are nested within the T-gasket G, the molds are not allowed to move axially relative to each other. Different T-gaskets G are required to form varying power lenses because each T-gasket G sets a predetermined axial separation between molds M. That is, one T-gasket G sets the molds farther apart to form a lens of a greater power compared with another T-gasket G used to form a lower power lens. Accordingly, manufacturers must maintain separate T-gaskets for a +2 lens, another for a -3 lens, another for a -4 lens, etc. In other words, in order to meet the requirements of various possible types and powers of lenses, a portfolio of different T-gaskets over a wide range of different geometrical characteristics is needed. Not only is it costly to create and keep such a portfolio, but even with a portfolio it is sometime still impossible to meet timely a customer's need because that customer may require a lense with power or type outside the portfolio, and then a customer-tailored T-gasket has to be made.
Second kind of closure member has been proposed as an alternative. Namely, a closure member can be made by wrapping a strip around the edges of the molds and thus a molding cavity can be created on demand in situ at the last minute by the molds and the strip. A number of references disclose various ways to achieve this goal. One group of references disclose lens casting using two molds joined together using a strip by gluing or using adhesive tape to attach the strip to the molds to form a molding cavity. U.S. Pat. No. 5,213,825 issued to Shimizu et al., U.S. Pat. No. 4,497,754 issued to Padoan, U.S. Pat. No. 3,211,811 issued to Lanman, and U.S. Pat. Nos. 5,547,618 and 5,662,839 issued to Magne are some examples in this category. One drawback related to this approach is that traces of adhesives may remain on the molds and on the finished lens and, hence, a specific cleaning procedure must be utilized to clean up the adhesives. Moreover, because the molds are held firmly by adhesives once the molds are assembled and the molding cavity is formed, the molds cannot move relative to the tape, or within the tape during the lens casting process.
Avoiding the drawbacks caused by adhesives, another group of references disclose lens casting simply using a strip of heat shrinkable synthetic material or similar deformable material to cooperate with the molds to form a molding cavity. For examples, U.S. Pat. No. 2,406,361 issued to Fairbank et al. and U.S. Pat. No. 5,611,969 issued to Cano et al. both disclose just using such tape to form the cavity into which monomer is injected. While this approach may have succeeded in avoiding the drawbacks caused by adhesives, it also does not allow the molds to move relative to the tape or within the tape once the molds are assembled and the molding cavity is formed. Therefore, to compensate any shrinkage of the lens forming material during the lens casting process, the tape used in these references must crumple or wrinkle, as clearly shown in FIG. 10 of the Cano patent and explained in Col. 4, lines 17-22 of the Fairbank patent. However, because the tape may not be crumpled or wrinkled uniformly under the pressure of the molds during the lens polymerization process, it may negatively impact the quality of the lens to be molded. Moreover, because the tape is heated to shrink it onto the molds tightly, separating the tape from the molds once the lens is made requires special care and extra effort, which increases labor cost and impedes automation of the molding process. Furthermore, all cited references which disclose using the tape arrangement do not have the flexibility to adjust the relative axial positions of the molds once the molds are closed with the tape. If a mistake is made in setting up an appropriate axial positions of the molds, the tape has to be unwrapped and the whole process has to start over again.